The Effects of Strain Induced Softening on the Load Carrying Capability of Components
by
James L. Handrock
Abstract
Recently interest has grown in using quenched and tempered 2.25 Cr 1 Mo steel in pressure vessels and piping systems. This material has a bainitic microstructure with improved short term strength characteristics over the more commonly used annealed form. The bainitic material cyclically softens, however, losing as much as 30 percent of its initial strength in a few hundred cycles at post yield strain ranges. In most practical instances, though, significant stress reversals in components only occur in localized regions, suggesting that the use of the enhanced strength characteristics would be acceptable.
In this study the effect of material softening on the load carrying capability of components was investigated. An elevated temperature (565°C) experimental test program was conducted using smooth, Bridgman notched, and British Standard V-notched test specimens. The basis of this program had a matrix of creep, fatigue, notch acuity, and stress ratio effects. From these test results a first-order model of cyclic softening was developed in which the current strength was defined as a function of the load cycle number and frequency. This model was then used to predict the results of completely reversed load control tests on both the plain and notched bars. Observations were also made regarding strain rate, frequency, and hold time effects.
A time independent uniaxial constitutive model was developed in which strain induced cyclic softening was defined as a function of accumulated plastic strain. Good predictions were made of both overall strength loss and hysteresis behavior under controlled strain range conditions. This constitutive model was then used to investigate the geometrical effects of softening in a two-bar structure.
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